1. Field of Invention
This invention pertains generally to neutron detection and, more particularly, to a neutron detector and method utilizing a material which emits light when acted upon by thermal neutrons.
2. Related Art
Neutrons are difficult to detect because, being uncharged elementary particles, they do not ionize matter as they pass through it. It is, therefore, necessary to rely upon some other form of physical reaction to indicate the presence of neutrons.
One area in which the detection of neutrons is employed is neutron radiography where thermal neutrons are used as the penetrating radiation to produce images of parts or materials on film. Thermal neutrons are produced by fissioning, or splitting, Uranium-235 atoms or other fissionable atoms in a nuclear reactor, following which the velocity of the neutrons is reduced to very slow, or "thermal", energies. The thermal neutrons are passed through the part or material to be examined.
Unlike X-rays which sensitize radiographic film directly, neutrons pass through the film without substantial interaction with the emulsion. Therefore, the film is backed with a conversion screen having a thin film of a material such as gadolinium which absorbs the neutrons, becomes radioactive and sensitizes the emulsion by the emission of radiation in the form of light and/or electrons. Other materials which fluoresce or scintillate when struck by thermal neutrons and have heretofore been utilized in conversion screens for neutron radiography include gadolinium oxysulfide and lithium fluoride.
The materials heretofore utilized as thermal neutron detectors have certain limitations and disadvantages. High quality gadolinium screens are a specialized product which in recent years have become extremely difficult to obtain. Also, with gadolinium screens, there is a problem of stray film darkening from gamma rays which result from the gadolinium capture of neutrons.